The long-lived phosphorescence refers to a phenomenon that a substance absorbs light, stores the light as energy, and then continuously emits light for a while even after the light is extinguished. Conventionally, there have been known long-lived phosphorescent materials formed of inorganic compounds as materials that exhibit such a strong and long-lived phosphorescence at ordinary temperature as to be recognizable with the naked eye. However, there have not been known long-lived phosphorescent materials formed of organic compounds.
Some organic compounds have been known to exhibit a phenomenon called phosphorescence. The phosphorescence is defined as luminescence based on spin-forbidden transition from an excited triplet state to a ground singlet state. In this phenomenon, afterglow is observed even after light irradiation is stopped. However, the lifetime of the phosphorescence is approximately 0.001 seconds to several seconds. To exhibit such a strong and long-lived (0.1 seconds or more) phosphorescence as to be recognizable with the naked eye, the organic compounds need to be brought into an extremely low temperature state of approximately 77 K. Organic materials are composed of light elements (carbon, hydrogen, oxygen, nitrogen) and are weak in intermolecular force and the like. Hence, interatomic stretching vibrations or rotational motions occur locally in the organic materials in a solid state at or below the glass transition temperature (Tg), as well. For this reason, although a phosphorescent dye exhibits a long-lived phosphorescence recognizable with the naked eye at an extremely low temperature of approximately 77 K, the phosphorescent dye excited at ordinary temperature is deprived of the excitation energy by thermal vibrations of the surroundings in at most several milliseconds. As a result, the phosphorescence with a long lifetime (long-lived phosphorescence) is almost unobservable (for example, refer to Non-Patent Document 1). Moreover, since oxygen exists in an organic material in air, the energy of the excited state is removed by the oxygen, and hence the excited state is unable to be kept for several milliseconds or longer. For this reason, a strong and long-lived phosphorescent phenomenon of an organic material at ordinary temperature in air has not been reported so far.
Meanwhile, long-lived phosphorescent materials formed of inorganic compounds are used for luminous paints and the like, because the materials have long lifetimes of long-lived phosphorescence even at ordinary temperature. However, such long-lived phosphorescent materials formed of inorganic compounds generally use Re4+:Cs2ZrBr6 or the like. Hence, such long-lived phosphorescent materials are not preferable because of the use of environmentally hazardous elements or rare metals. Recently, those using Mn2+:Ta2O5, Dy:CuZnS or Eu:SrAlO4 and being relatively harmless to the human body have been commercialized (refer to, for example, Patent Document 1). However, with such long-lived phosphorescent materials formed of inorganic compounds, it is difficult to perform a crystalline-amorphous control by heat, and to change the crystal structure by light. A heat storage function is unable to be reversibly recorded or controlled by practical heat application or practical light irradiation.
Note that, with an organic material, rewriting through a crystalline-amorphous control by heat, rewriting by light using a photochromic compound, or the like may be performed. Actually, the present inventors had developed a recording medium in which fluorescence portions are thermally rewritable (Non-Patent Document 2 and Patent Documents 2 and 3). However, this is a material having a fluorescence function (in which the luminescence stops immediately after the irradiation of excitation light is stopped), but not an organic material having a long-lived-phosphorescence function at ordinary temperature.
In this respect, there has been desired development of a long-lived phosphorescent material formed of an organic compound having various characteristics, which inorganic compounds do not have, such as capability of the reversible recording by phase transition.    Non-Patent Document 1: K. Horie, K. Morishita, and I. Mita, Macromolecules, 1984, vol. 17, pp. 1746    Non-Patent Document 2: Shuzo Hirata and Toshiyuki Watanabe, Advanced Materials, 2006, vol. 18, Issue 20, pp. 2725-2729    Patent Document 1: Japanese Patent No. 2543825    Patent Document 2: Japanese Patent Application Publication No. 2007-118420    Patent Document 3: International Patent Application Publication No. WO2007/111298